Method for depositing electrically conducting polymer films via electrochemical deposition of precursor polymers

ABSTRACT

A method for depositing an electrically conducting film on an electrode and the film resulting from that method. An electrically conducting film according to the present invention is deposited by immersing the electrode in a solution of a precursor polymer in a predetermined solvent. The precursor polymer includes a plurality of electrochemical polymerizable monomers. Each monomer has first and second polymer-forming active sites that can be joined by electrochemical polymerization and third and fourth polymer-forming active sites that can be joined chemically in solution. The precursor polymer is constructed from the monomers joined by the third and fourth polymer-forming active sites. The precursor polymer is soluble in the solvent whereas a polymer formed by electrochemical polymerization of the first and second polymer-forming active sites is insoluble in the solvent.

FIELD OF THE INVENTION

The present invention relates to polymer-based electrically conductingor electroluminescent films, and more particularly, to films constructedfrom a class of precursor polymers that are electrochemicallypolymerized to form the electrically conducting film.

BACKGROUND OF THE INVENTION

Polymer-based electroluminescent devices (PLEDs) have the potential forproviding inexpensive alternatives to alpha-numeric displays and x-yaddressable displays. PLEDs also have the potential to provide analternative to back lighted, liquid crystal displays. A simple PLED maybe constructed from an electroluminescent layer sandwiched between anelectron injection electrode and a hole injection electrode. Theelectroluminescent layer is typically constructed by depositing aconjugated or conductive polymer on one of the electrodes. Devices basedon poly(p-phenylenevinylene) (PPV), or derivatives thereof, have beendemonstrated with sufficient quantum yields to be commerciallyattractive. More complicated devices utilize electron and hole transportlayers between the above mentioned electrodes and the electroluminescentlayer. The electroluminescent layer generates light when holes andelectrons recombine in the layer.

The deposition and patterning of the electroluminescent layer presentsignificant technical problems that must be overcome before economicallyattractive devices can be fabricated. If the conjugated polymer issoluble in a solvent, a thin film can be made by the spin-coating of apolymer solution. While spin-coated polymer films having goodelectro-optical properties can be obtained in this manner, the adhesionof spin-coated film to the underlying layer is often insufficient. Inaddition, many attractive polymers are not sufficiently soluble to beapplied via spin-coating.

Spin-coating and other processes in which the entire substrate iscoated, present additional problems in multi-color displays in whichdifferent “pixels” must be coated with different polymers. Thedeposition of each layer requires a three-step procedure consisting of amasking step to protect areas that are not to be coated, thespin-coating step, and a mask removal step. In addition to the increasedcomplexity of the masking steps, the solvents utilized with conventionalmasking systems are often incompatible with the polymers beingdeposited. Accordingly, it would be advantageous to provide a systemthat does not require such masking operations.

Broadly, it is the object of the present invention to provide animproved method for depositing an electrically conducting orelectroluminescent film.

It is another object of the present invention to provide a method thatmay be utilized with materials that cannot be spin-cast.

It is further object of the present invention to provide a method thatcan selectively deposit such films without the use of the maskingoperations discussed above.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention is a method for depositing an electricallyconducting film on an electrode and the film resulting from that method.An electrically conducting film according to the present invention isdeposited by immersing the electrode in a solution of a precursorpolymer in a predetermined solvent. The precursor polymer includes aplurality of electrochemical polymerizable monomers. Each monomer hasfirst and second polymer-forming active sites that can be joined byelectrochemical polymerization and third and fourth polymer-formingactive sites that can be joined chemically in solution. The precursorpolymer is constructed from the monomers joined by the third and fourthpolymer-forming active sites. The precursor polymer is soluble in thesolvent whereas a polymer formed by electrochemical polymerization ofthe first and second polymer-forming active sites is insoluble in thesolvent. An electrical potential is applied to the electrode to causemonomers of the precursor polymer molecules to be joinedelectrochemically by their first and second polymer-forming activesites. Precursor polymers may be constructed from monomers such asfluorene, thiophene, pyrrole, biphenyl, poly(vinyl carbazole) or poly(vinyl oxy thiophene). Precursor polymers may also be constructed fromdimers constructed from monomers chosen from this group. The monomersmay include a spacer group bonded to one of the first or second activesites. Exemplary spacer groups include (CH₂)_(n), (OCH₂)_(n), or(OCH₂CH₂)_(n), where 1<n<20. The solution may also includenon-polymerized monomers or other compounds that can beelectrochemically linked to the first and second polymer-forming activesites of the monomers. The concentration of the monomers by molarpercent of monomeric repeat unit of polymer is between 0.01 and 99.99%.These monomeric units are joined to the first and second polymer-formingactive sites of the monomers in the precursor polymers, and to eachother, during the electrochemical polymerization thereby forming chainsof the monomeric units that are linked to the precursor polymers by thefirst or second polymer-forming active sites of the monomers in theprecursor polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the polymerization of monomer units via two differentprocesses.

FIG. 2 is an example of a flexible spacer cross-linking between twochains of monomers.

FIG. 3 is an example of a cross-linking between two chains of monomersvia an electroluminescent active site.

FIG. 4 shows some examples of monomeric functional groups that may beutilized to form the precursor polymers of the present invention.

FIG. 5 illustrates the preparation of one class of electroluminescentpolymers according to the present invention.

FIG. 6 is a schematic representation of two possible polymers formedfrom monomers labeled A and B.

FIG. 7 illustrates the electrochemical polymerization of a precursorpolymer having the configuration shown at 61 in FIG. 6 on an electrode.

FIG. 8 is an example of the formation of an electroluminescent polymerlayer based on a dimer unit such as shown in FIG. 6.

FIG. 9 illustrates the various types of chemical structures that areobtained when a solution having precursor polymers and various monomersis electrochemical polymerized.

FIG. 10 is a top view of a display according to the present invention.

FIG. 11 is a cross-sectional view of the display shown in FIG. 10through line 251-252.

FIGS. 12-14 are top views of a display 200 at various stages in thefabrication process.

DETAILED DESCRIPTION OF THE INVENTION

The manner in which the present invention achieves its advantages may bemore easily understood with reference to FIG. 1, which is a schematicdrawing of the polymerization of monomer units 11 via two differentprocesses. Monomer 11 is chosen such that it has four functional groupsthat can be utilized to form polymers. Groups 12 and 13 can be linkedwith the aid of a spacer 16 to form a polymer 21 that is soluble in apredetermined solvent, but which is neither conducting norelectroluminescent. Groups 14 and 15 can be linked via electrochemicalpolymerization to form either an electroluminescent or electricallyconducting polymer 22, which is insoluble in the solvent. Since polymer22 is insoluble, the number of units that can be linkedelectrochemically is limited. As a result, the quality of polymer filmsobtained by electrochemical polymerization groups 14 and 15 leaves muchto be desired. In many cases, the film consists of a layer of powderymaterial that is precipitated in the vicinity of the electrode ratherthan being deposited onto the electrode. This precipitation problemresults from the poor solubility of polymer units having a few monomerslinked together that are formed in the vicinity of the electrode.Attempts to overcome this problem by utilizing parent monomers withflexible-side groups to increase the solubility of short chain polymerunits have not yielded films of a quality needed for electronic devices.In addition, this approach also has a disadvantage of reducing theconcentration of electroactive functional groups in the film structure,and hence, the efficiency of light generation by the films.

The present invention avoids the problems associated with polymer 22 byutilizing polymer 21 as the precursor for forming an electricallyconducting or electroluminescent polymer 23 by electrochemicalpolymerization of polymer 21. The spacers 16 in polymer 21 provide ahigh degree of connectivity to the moieties contain groups 13 and 14,and hence, result in the deposited film having a higher degree ofmechanical strength and uniformity than the films obtained by directelectrochemical polymerization of the unlinked moieties. As a result,the polymer is deposited on the electrode and can grow into a highquality film.

The film formed during the electrochemical polymerization may betwo-dimensional, or even, three-dimensional because of the cross-linkingof the various one-dimensional polymer chains. The cross-linking occursthrough the sites used to link the flexible spacers or through theelectroluminescently active sites. An example of a flexible spacercross-linking between two chains 31 and 32 is shown in FIG. 2. Thecross-link is shown at 33. This type of cross-linked structure is formedduring the preparation of the precursor polymers. In this case, thepi-conjugated bonds are formed during electrochemical polymerizationbetween monomers in the same chain.

An example of a cross-linking between two chains 41 and 42 via api-conjugated bond 43 is shown in FIG. 3. In this case, the chains arelinked during the electrochemical polymerization. It will also beapparent from the previous discussion that more complex arrangements maybe formed in which two or three-dimensional chains are furthercross-linked during either the original polymerization using theflexible spacers or the electrochemical polymerization.

Some examples of monomeric functional groups that may be utilized toform the precursor polymers are shown in FIG. 4 at 301-306. Compounds301-306 are fluorene, thiophene, pyrrole, biphenyl, poly(vinylcarbazole) and poly (vinyl oxy thiophene), respectively. These groupsmay be joined by spacers of the form (CH₂)_(n), (OCH₂)_(n),(OCH₂CH₂)_(n), . . . , where 1<n<20.

Refer now to FIG. 5, which illustrates the preparation of one class ofelectroluminescent polymers according to the present invention. A seriesof precursor polymers was prepared by dilithiation of fluorene withn-butyllithium, followed by the reaction with α,ω-dibromoalkanes(n=4,6,8,10,12). The formed precursor polymers are soluble in commonorganic solvents such as chloroform, dichloromethane or toluene.

Electrochemical polymerization is carried out using solutions ofprecursor polymers at a concentration of 50 mM with electrolyte(tetrabutylammonium tetrafluoroborate: 100 mM) in dichloromethane as asolvent. Conductive glass substrates with a thin layer of indium-tinoxide may be used as working electrodes. Cyclic voltammetry is performedfor each precursor polymer solution, between −200 mV and +2000 mV(versus Ag/Ag+ reference electrode). At the first cycle, the oxidationof the fluorene group is observed at +1350 mV. At the following cycles,reversible oxidation and reduction is observed at around +1200 mV, andthe change in color of the material on the electrode is observed (lightbrown at low potential, bright red at high potential). After the filmhas reached the desired thickness, the electrodes are rinsed withtoluene, which is a good solvent for these precursor polymers to removeany non-electrochemically polymerized precursor polymers. A thin film ofa light-brown material remains on the electrode surface. This filmexhibits an absorbence spectra that is very close to that ofpoly(fluoren-2,7-diyl).

The precursors of the present invention can also be utilized to formfilms of copolymers from monomers having different electroniccharacteristics, such as emission spectra and energy bands. Refer now toFIG. 6, which is a schematic representation of two possible polymersformed from monomers labeled A and B. To simplify the discussion, unit Ais assumed to be the same as monomer unit 11 shown in FIG. 1. Unit B,which is shown at 51, also has two active sites, 52 and 53, that may beused to polymerize the monomers to form a soluble chain that does notemit light and two active sites, 54 and 55, that can be joined viaelectrochemical polymerization to form a light emitting polymer that isinsoluble in the solvent. The monomeric units can be combined to form asoluble copolymer precursor in a number of different configurations. Forexample, a precursor block copolymer 61 can be generated by combiningthe A units via spacer 56 to form a poly-A precursor that is then joinedto a poly-B precursor formed by joining the B monomers via spacers 59.Similarly, the two sub-units can be joined via a spacer 57 to form thealternating copolymer precursor. A second configuration for theelectrochemical polymerization precursor is shown at 62. In this case, adimer consisting of one A-unit and one B-unit is first formed. Thedimers are then polymerized with the aid of spacers 58 to form thealternative copolymer precursor.

While the examples shown in FIG. 6 utilize different spacers, it will beobvious to those skilled in the art from the preceding discussion thatthe monomers may be chosen such that a single spacer unit can beutilized for all of the sub-unit attachments. A random copolymerprecursor polymer may be constructed by applying the solutionpolymerization process to a solution containing both monomers. It willalso be apparent form the preceding discussion that precursors havingmore than two types of monomers may also be constructed in this manner.For example, the monomeric functional groups can be two or more of anyelectrochemically polymerizable group such as fluorene, thiophene,pyrrole, phenylene, pyridine, or triptycene or one of the derivatives ofthe above compounds. Component of each species can be between 0.001% and99.999%.

The precursor copolymer can have any form of structure includingalternative copolymer, block copolymer, graft copolymer and randomcopolymer. Furthermore, the precursors can form complex two andthree-dimensional structures as discussed above. The functional groupscan be either linked by a spacer or simply attached to a polymerbackbone. The spacer or polymer backbone can be any flexible molecularchains such as oligomethylene units or oligooxymethylene units, or itmay contain functional groups such as phenyl group or amino group. Thenumber of repeating units either in the spacer or between electroactivefunctional groups is preferably between 1 and 20.

Once the precursor polymer is prepared, the precursor polymer isdissolved in a solvent with an electrolyte. A cyclic voltammogram or abulk electrolysis can be carried out for this solution. By choosing theappropriate potential of the electrode for each functional unit, theprecursor polymer is polymerized and deposited on the electrode. Refernow to FIG. 7, which illustrates the electrochemical polymerization of aprecursor polymer having the configuration shown at 61 in FIG. 6 on anelectrode 70. For the purposes of this discussion, it will be assumedthat the A-subunit has a lower oxidation potential than the B-subunit.Hence, the A-subunits will be joined at the lower potential differencebetween the electrode on which the film is deposited and the referenceelectrode. This leads to a film in which the A-subunits are polymerizedas shown at 72 in FIG. 7. When all of the A-subunits have beenpolymerized, the B-subunits will then be oxidized to form the polymershown at 73.

An electroluminescent polymer layer based on a dimer unit such as shownin FIG. 6 at 62 may be formed in the manner shown in FIG. 8. A precursorpolymer 81 is prepared by dilithiation of fluorene with n-butyllithium,followed by the reaction with 3,4bis(ω-bromoalkyl) thiophene. Polymerswith n=4,6,8,10,12 can be utilized; however, precursors with other nvalues may also be useful. The formed precursor polymers are soluble incommon organic solvents such as chloroform, dischloromethane, ortoluene.

Electrochemical polymerization can then be carried out using solutionsof precursor polymers at a typical concentration of 50 mM indichloromethane as a solvent to which an electrolyte (tetrabutylammoniumtetrafluoroborate: 100 nM) has been added. Conductive glass substrateswith a thin layer of indium-tin oxide can be used as working electrodes.The electrochemical polymerization is carried out via cyclic voltammetrytypically performed between −200 mV and +2000 mV (versus Ag/Ag+reference electrode). At the first cycle, the oxidation will beobserved, due to oxidation of thiophene and fluorene, respectively. Atthe following cycles, reversible oxidation and reduction will beobserved at around +1000 mV and +1200 mV, and the change in color of thematerial on the electrode will also be observed. After thepolymerization is completed, the electrodes are rinsed with toluene, toremove any unpolymerized precursor polymers. A thin film of a reddishmaterial will remain on the electrode surface. This thin film shouldexhibit an absorbency spectra that has the combined characteristics ofpoly(thiophen-2,5diyl) and poly(fluoren-1,7diyl).

It should be noted that the precursor polymers described above may alsobe used as “seed” polymers for growing additional electroluminescent orconducting chains. It has been found experimentally that the seedpolymers may be mixed with the monomers used to form the precursor aswell as other monomers that have only electrochemical polymerizationactive sites. Refer now to FIG. 9, which illustrates the various typesof chemical structures that are obtained when a solution havingprecursor polymers and various monomers is electrochemical polymerizedon an electrode 91. In this example, the precursor polymers areconstructed from a monomeric unit A, an exemplary precursor polymerbeing shown at 96. Two types of monomers are assumed to be present inthe solution, monomers of type A as shown at 94 and monomers of type Dshown at 95. For example, the D monomers may be dyes that determine theemission characteristics of the spectral lines emitted by the finalelectroluminescent layer that is deposited on electrode 91.

As noted above, the precursors form an insoluble material by formingbonds between adjacent A's in the precursor such as shown at 99 and bycross-linking two precursor polymers such as precursor polymers 92 and93. The various monomers can be electrochemical polymerized onto any ofthe open active sites on the precursor polymers that permit suchpolymerization. Once a monomer has bonded to a precursor polymer, it canprovide the starting point for a chain of monomers such as shown at 97and 98.

A high quality electrically conduction and/or electroluminescent layercan be obtained in this manner with a relatively low percentage ofprecursor polymers. It has been found experimentally that precursorpolymer concentrations as low as 1% can provide high quality films inthe presence of 99% monomers. For example, a high quality conductingfilm was deposited in this manner by the electrochemical polymerizationof a solution having 100 Moles of oxlylthiophene per mole of therepeating unit in the polybinyl carbozole.

The films generated by the electrochemical polymerization of theprecursor polymers in the presence of the monomer have two advantagesover films obtained by the electrochemical polymerization of justprecursor polymers. First, the resultant polymer can have a higherdensity of electroactive active sites than that obtained withoutincluding the monomer units, and hence, provides superior light output.Films may be deposited with as little as 0.01% precursor polymer and99.99% monomer. Second, the cost of the film is substantially reduced,since the cost of making the precursor can be quite high.

It should be noted that the film can be deposited in two steps. In thefirst step, the precursor polymers are deposited via electrochemicalpolymerization forming a three-dimensional mesh. In the second step, themonomers, including any dyes are deposited by electrochemicalpolymerization on the precursor mesh. In this embodiment of the presentinvention, the surface with the precursor deposited film can be providedas a starting material that is customized for a particular applicationby second electrochemical polymerization.

The precursor polymers of the present invention are well suited forconstructing multi-color displays such as the pixelated display shown inFIGS. 10 and 11. FIG. 10 is a top view of a display 250, and FIG. 11 isa cross-sectional of display 250 view through line 251-252. Display 250is composed of a number of pixels arranged in a rectangular array havingrows and columns defined by horizontal electrodes and verticalelectrodes. An exemplary horizontal electrode is shown at 253, and anexemplary vertical electrode is shown at 254. An electroluminescentpolymer 255 is sandwiched between the horizontal and vertical electrodesat each intersection point. An individual pixel is activated by applyinga voltage between the row and column electrodes that define that pixel.In the example shown in FIGS. 10-11, the vertical electrodes aredeposited on a substrate 256 by patterning a conducting layer that isdeposited on the substrate.

The manner in which the precursors of the present invention are used toconstruct a pixelated multi-colored display such as that shown in FIGS.10-11 may be more easily understood with reference to FIGS. 12-14, whichare top views of a display 200 at various stages in the fabricationprocess. In this example, the display is constructed from red, blue, andgreen pixels that are addressed by the row-column addressing schemediscussed above. To simplify the following discussion, it will beassumed that each pixel consists of an electroluminescent polymer layersandwiched between the two electrodes. The pixels are fabricated on thecolumn electrodes by depositing the electroluminescent polymer layersconstructed from precursors according to the present invention for thevarious colors on the corresponding column electrodes electrodes. Therow electrodes are then deposited. For the purposes of this discussion,it will be assumed that the column electrodes have already beenpatterned on a substrate 201 as shown in FIG. 12. For example, thecolumn electrodes can be patterned from indium tin oxide that islithographically patterned on a glass substrate. It will also be assumedthat any additional layers such as an electron transport layer that arecommon to all pixels have also been deposited.

The deposition of the electroluminescent material is carried out inthree separate electrochemical deposition steps, one for each color. Thecolumn electrodes that are to receive the red, green, and blue-emittingmaterials are shown at 202-204, respectively. The red-emitting polymerlayer is deposited by connecting all of the red columns 202 together andplacing the substrate in a deposition bath 210 that contains theprecursor polymers and dyes for the red-emitting layer as shown in FIG.12. The precursors and monomers are polymerized as discussed above.During the deposition of the red polymer, the green and blue columns arenot energized; hence, material is only deposited on the red columns 202.At the end of the deposition process, the red-emitting columns arecoated with a polymer film 212 as shown in FIG. 11.

The process is then repeated by connecting columns 203 together andutilizing an electrochemical bath having the precursor polymers and dyesfor the green-emitting layer. Finally, the process is repeated byconnecting columns 204 together and utilizing an electrochemical bathhaving the precursor polymers and dyes for the blue-emitting layer.

It should be noted that the column electrodes can be patterned such thatall of the electrodes that are to receive a particular color areconnected by bus lines that run over a portion of the substrate that isnot utilized in the final display. These bus lines can then be removedafter the electrochemical deposition is complete by removing thatportion of the substrate or by breaking the connections. For example,the traces can be broken by laser ablation of the indium tin oxide inselected areas.

The thickness of the film determines both the final light output of thedevice and, to some extent, the color of the emitted light. In thepresent invention, the thickness of the deposited film can be controlledfor each film layer by monitoring the thickness during deposition byoptical measurements such as low coherence reflectometry or by observingthe current flowing through the electrode during the deposition. Inaddition, the deposition system can be calibrated by using differentdeposition times and then measuring the light output of the resultingdevices.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

What is claimed is:
 1. An electrically conducting polymer filmcomprising a plurality of precursor polymers, each precursor polymercomprising a plurality of monomers, each monomer having first and secondpolymer-forming active sites that can be joined by electrochemicalpolymerization and third and fourth polymer-forming active sites thatcan be joined chemically in solution, each precursor polymer comprisingsaid monomers joined by said third and fourth polymer-forming activesites, at least two of said precursor polymers further being linked bysaid first and second polymer-forming active sites on monomers in thoseprecursor polymers, said precursor polymers being soluble in apredetermined solvent whereas said polymer film is insoluble in saidsolvent.
 2. The electrically conducting film of claim 1 wherein saidmonomers comprise fluorene, thiophene, pyrrole, biphenyl, poly(vinylcarbazole) or poly (vinyl oxy thiophene).
 3. The electrically conductingfilm of claim 1 wherein said monomers further comprise a spacer group,said third or fourth active sites being located on said spacer group. 4.The electrically conducting film of claim 3 wherein said spacer groupcomprises (CH₂)_(n), (OCH₂)_(n), or (OCH₂CH₂)_(n), where 1<n<20.
 5. Theelectrically conducting film of claim 1 wherein said monomers comprisefirst and second chemical species.
 6. The electrically conducting filmof claim 5 wherein said first and second chemical species occur randomlywithin said precursor polymer.
 7. The electrically conducting film ofclaim 5 wherein said precursor polymer comprises regions having aplurality of monomers of said first species coupled to each other toform a co-polymer.
 8. An electrically conducting polymer film comprisinga plurality of precursor polymers, each precursor polymer comprising aplurality of monomers, each monomer having first and secondpolymer-forming active sites that can be joined by electrochemicalpolymerization and third and fourth polymer-forming active sites thatcan be joined chemically in solution, each precursor polymer comprisingsaid monomers joined by said third and fourth polymer-forming activesites, at least two of said precursor polymers further being linked bysaid first and second polymer-forming active sites on monomers in thoseprecursor polymers, said precursor polymers being soluble in apredetermined solvent whereas said polymer film is insoluble in saidsolvent, wherein said monomers comprise a dimer constructed from twocompounds selected from the group consisting of fluorene, thiophene,pyrrole, biphenyl, poly(vinyl carbazole) and poly (vinyl oxy thiophene).9. An electrically conducting polymer film comprising a plurality ofprecursor polymers, each precursor polymer comprising a plurality ofmonomers, each monomer having first and second polymer-forming activesites that can be joined by electrochemical polymerization and third andfourth polymer-forming active sites that can be joined chemically insolution, each precursor polymer comprising said monomers joined by saidthird and fourth polymer-forming active sites, at least two of saidprecursor polymers further being linked by said first and secondpolymer-forming active sites on monomers in those precursor polymers,said precursor polymers being soluble in a predetermined solvent whereassaid polymer film is insoluble in said solvent, said electricallyconducting polymer film further comprising a chain of electricallypolymerizable monomeric units, said chain being coupled to one of saidprecursor polymers via one of said first or second polymer-formingactive sites of one of said monomers in that precursor polymer.
 10. Theelectrically conducting film of claim 9 wherein said chain comprises aplurality of said monomers joined by said first and secondpolymer-forming active sites.